Paint the center of the bit with molasses. Leave in for an hour. Remove. Repeat. Repeat. Repeat. After a while, they will practically open their mouth when they see it coming.
Tessy, horses are prey animals, if you trap them in a corner where they have no escape, they can become agressive because they have no options except fight. They can't feel comfortable in that situation, and a stressed horse won't learn. Always give the horse a way to escape if needed , because that's how they feel safe. A gentle horse could deal with it, but they also can panic and freaks out.
Put him in a stall and position him back towards a corner where he can't back up. Make sure you teach him "head down" or he may start throwing his head up to avoid the bit. Also make sure it's a very gentle baby type bit like a softish happy mouth. I used to know a trainer that put all young ones in a stall with bridle and bit on and leave them there for several hours every day til it was normal for them, but I wouldn't recommend that way anyway :)
I often start mine with a cookie in the hand that the bridle isn't in. So I hold the bridle up with the bit touching the teeth with my right hand and then hold a cookie just below the bit in my left hand. They're usually looking for the bit after the first few times and I remove the cookie element as soon as they do.